METHOD OF GROWING MOBILE POSITIVE ION FREE SiO{11 {11 FILMS

ABSTRACT

The present invention relates to a process for growing and annealing a silicon dioxide insulator layer to obtain a mobile positive ion free silicon dioxide insulator layer. A mobile positive ion free silicon dioxide insulator layer is required in order to make a stable insulated gate field effect transistor. The processing apparatus comprises a non-oxidizing, high-meltingpoint platinum metal film coated quartz furnace tube, potential means for placing a positive potential upon the platinum metal film coating of the platinum metal film coated quartz furnace tube to repel mobile ions therefrom, heater means for heating the interior of the quartz furnace tube, and gas means for passing oxygen gas through the platinum metal film coated quartz furnace tube. A silicon wafer may be oxidized in said processing apparatus to form a relatively mobile positive ion free silicon dioxide insulator layer of an insulated gate field effect transistor upon the silicon wafer. The silicon dioxide insulator layer is relatively uncontaminated by mobile positive ions which exist to the outside of the platinum metal film coated quartz furnace tube. A silicon wafer which previously has been coated by a silicon dioxide insulator layer may be processed to remove mobile ions within the silicon dioxide insulator layer. Mobile positive ions are repelled by the positive potential applied to the platinum metal film away from the outside of the quartz furnace tube, and mobile positive ions of the silicon dioxide insulator layer within the platinum metal film coated quartz tube are removed by the flowing oxygen gas due to the low vapor pressure of the mobile positive ions.

oepp et al.

[ 4] METHOD OF GROWING .MOnnE POSITIVE ION FREE $10 FILMS [75] Inventors: Ronald L. Roepp, Dayton; Stanley J. Dudkowslri, Kettering, both of Ohio [73] Assignee: The National Cash Register Company, Dayton, Ohio 22 Filed: Aug. 5, 1971 21 Appl.No.:169,546

Related U.S. Application Data [62] Division of Ser. No. 866,185, Oct. 14, 1969, Pat. No.

[52] U.S. Cl 117/201, 117/95, 117/106, 1l7/l07, 117/118, 117/227, 117/229 [51] Int. Cl. B44c l/18, C23c ll/00 [58] Field of Search..... 117/227, 95, 107, 229, 201, 117/118, 106 A; 23/277, 252 R; 13/1, 20; 118/48; 204/312 Primary ExaminerAlfred L. Leavitt Assistant ExaminerM. F. Esposito Attorney, Agent, or FirmJ. T. Covender et a1.

[57] ABSTRACT The present invention relates to a process for growing [111 3,787,233 [451 Jan. 22, 1974 and annealing a silicon dioxide insulator layer to obtain a mobile positive ion free silicon dioxide insulator layer. A mobile positive ion free silicon dioxide insulator layer is required in order to make a stable insulated gate field effect transistor. The processing apparatus comprises a non-oxidizing, high-melting-point platinum metal film coated quartz furnace tube, potential means for placing a positive potential upon the platinum metal film coating of the platinum metal film coated quartz furnace tube to repel mobile ions therefrom, heater means for heating the interior of the quartz furnace tube, and gas means for passing oxygen gas through the platinum metal film coated quartz furnace tube. A silicon wafer may be oxidized in said processing apparatus to form a relatively mobile positive ion free silicon dioxide insulator layer of an insulated gate field effect transistor upon the silicon wafer. The silicon dioxide insulator layer is relatively uncontaminated by mobile'positive ions which exist to the outside of the platinum metal film coated quartz furnace tube. A silicon wafer which previously has been coated by a silicon dioxide insulator layer may be processed to remove .mobile ions within the silicon dioxide insulator layer. Mobile positive ions are repelled by the positive potential applied to the platinum metal film away from the outside of the quartz furnace tube, and mobile positive ions of the silicon dioxide insulator layer within the platinum metal film coated quartz tube are removed by the flowing oxygen gas clue to the low vapor pressure of the mobile positive Ions.

5 Claims, 5 Drawing Figures Pmmmmze w 3.787. 233

METHOD OE GROWING MOEIEE POSITIVE ION 'EEEE SIOZIITILM CROSS REFERENCE TO RELATED APPLICATION:

This application is a division of co-pending application Ser. No. 866,185, filed on Oct. 14, I969 assigned to the same assignee of this invention and now U. S. Pat. No. 3,645,695.

BACKGROUND OF THE INVENTION United States Pat. No. 3,380,852, issued Apr. 30, 1968, on the application of Adolf Goetzberger, discloses a method of forming a relatively uncontaminated silicon oxide layer upon a silicon wafer, comprising placing a silicon wafer within a quartz furnace tube, placing a wire electrode in close spatial relation above the silicon wafer, applying a positive potential to the electrode with respect to the silicon wafer, and oxidizing the silicon wafer with the positive potential applied to the electrode. Goetzberger states that charges from the positively-charged electrode neutralize negative ions within the silicon dioxide layer, so as to form a relatively uncontamined silicon dioxide layer.

Goetzberger does not disclose a metal film coated quartz furnace tube having a positive potential applied to the metal film to repel positive ions away from the exterior of the metal film and to allow a flowing oxidation gas to absorb positive ions from within a silicon dioxide layer. Goetzberger has only a penetrable positive potential around a silicon wafer, by setting up an electric field between a positively charged wire electrode and the negatively-charged wafer. In the processing apparatus of the present invention, a potential difference is not set up between the metal film coating of a metal film coated quartz furnace tube and a silicon wafer, but the silicon wafer exists within a positive equipotential closed metal film. The silicon wafer is not subjected to an electric field during its oxi dation or during its annealing, but both during oxidation and during annealing, positive ions that may exist to the outside of the metal film coated quartz furnace tube are repelled and prevented from entering the metal film coated quartz furnace tube, as a result of a positive potential existing upon the metal film coating of the metal film coated quartz furnace tube with respect to ground. Therefore positive ions are prevented from diffusing through the metal coated quartz furnace tube and combining with a silicon dioxide insulator layer during its formation upon the silicon wafer. Positive ions are also prevented from diffusing through the metal film coated quartz furnace tube to combine with a silicon dioxide insulator layer during its annealing. During annealing of the silicon dioxide insulator layer, positive ions emitted from the silicon dioxide insulator layer are absorbed in a flowing oxidation gas which is passed through the interior of the metal film coated quartz furnace tube.

The positively-charged wire electrode of Goetzberger repels some positive ions that exist on the outside of the area between the positively-charged wire electrode and the negatively-charged silicon wafer, but does not repel positive ions which get into said area. In fact, positive ions which get into said area are absorbed by the negatively-charged silicon wafer.

The processing apparatus of the present invention hinders the combination of positive ions which may enter within the metal-film-coated quartz furnace tube with a silicon dioxide insulator layer therein, due to the flushing of the metal-film-coated quartz furnace tube with oxygen gas.

. SUMMARY OF THE INVENTION DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of the processing apparatus of the present invention used as an annealing furnace.

FIG. 2 is a cross-sectional view of an externally coated processing furnace tube.

FIG. 3 is a cross-sectional view of an internally coated passivated processing furnace tube.

FIG. 4 is a side view, with parts broken away, of the processing apparatus of the present invention used as an oxidation furnace.

FIG. 5 is a cross-sectional view of an MOS field effect transistor having a positive sodium ion free silicon dioxide insulator layer.

DESCRII TION OF THE PREFERRED EMBODIMENT FIG. 1 shows a processing apparatus for annealing a silicon dioxide insulator layer to deplete the concentration of positive ions therein. As shown in FIG. 1, a 0.5-centimeter-thick furnace tube 12 has evaporated upon its inner surface 13 a 700-Angstrom-thick nonoxidizing, high-melting-point metal film 14 to form an inner wall therein. The metal film 14 is preferably a platinum or rhodium film. However, a tantalum or titanium film, with an oxygen-impervious silicon nitride layer thereon, can be used. The metal film 14, even though being an evaporated amorphous film on the furnace tube 12, is a good barrier to the passage of mobile ions, such as mobile positive sodium ions, due to its crystal structure.

In accordance with the present invention, a metal film 14 has been found to be necessary to make a resistively heated fumace tube, such as a quartz, aluminum oxide, silicon carbide, or silicon nitride furnace tube, impervious to ions, such as positive sodium ions, at high temperature. A resistively-heated furnace tube 12 will reach a temperature of approximately l,l00 Centigrade, at which temperature it is porous to positive sodium ions. That is, sodium ions are highly mobile in diffusing through the crystalline structure of the furnace tube 12.

A platinum fihn 14 should be as thick as 600 Angstroms to stop most of the mobile ions from getting therethrough. However, a thinner film would be fairly effective in stopping mobile ions. An evaporated platinum film 14 which is made thicker than 600 Angstroms is increasingly effective in stopping mobile ions. An evaporated platinum film 14 which is thicker than 6,000 Angstroms begins to peel from the quartz furnace tube 12. Therefore, a platinum film 14 which is between 600 Angstroms and 6,000 Angstroms thick is preferred.

As shown in FIG. 2, a platinum film 8 may be evaporated on the outside of a quartz furnace tube 6. Such a platinum film 8 which is between 600 and 6,000 Angstroms thick is effective in stopping mobile ions from entering the interior of the quartz furnace tube 6. However, since mobile ions are located in the quartz furnace tube 6 itself, it is preferable that a platinum film be on the inside of the quartz furnace tube 6.

In FIG. 1, the platinum film 14, being on the inner surface of the furnace tube 12, hinders the mobile positive sodium ions within the wall of the furnace tube itself from diffusing into the interior of the furnace tube 12 at high temperature. The platinum film 14 does not melt at l,lO Centigrade and does not oxidize at l,lO0 Centigrade in an oxidizing atmosphere. Gold, which also is a metal, melts near l,063 Centigrade. Therefore, gold cannot be used to coat a quartz furnace tube 12 which is heated to l,lO0 Centigrade. Silver also melts below l,lO0 Centigrade. A copper film melts below l,lO0 Centigrade. Therefore a copper film may not be used to form a non-porous barrier to the passage of mobile ions into the interior of the quartz processing furnace tube 12 when it is used in a high temperature oxidation processing apparatus.

As shown in FIG. 3, a tantalum film 10 may be evaporated upon the inside surface of a quartz furnace tube 9. The tantalum film 10 oxidizes in an oxidizing atmosphere, at l,lO0 Centigrade. However, a layer 11 impervious to oxygen at high temperature, such as a silicon nitride layer, is placed upon the exposed surface of the 700-Angstrom-thick tantalum film 10. The silicon nitride layer 11 prevents oxygen in the interior of the tantalum-coated quartz furnace tube 9 from oxidizing the tantalum film 10. The tantalum film 10 on the inner surface of the quartz furnace tube 9 prevents mobile ions from entering the interior of the quartz furnace tube 9.-

A metal film is preferred for use in the processing apparatus of the present invention which has no mobile ions, has a high melting point, and does not oxidize at a high temperature. A platinum film is a very good metal film which has these properties. Its atomic crystalline structure is fine enough to stop mobile ions from passing through it at high temperature.

As shown in FIG. 1, the platinum film 14, having been evaporated, is an amorphous platinum film. The evaporation causes the platinum film, which is evaporated on the inside of a quartz tube, to be amorphous. However, the evaporated film 14, being semicrystalline, is impervious to mobile ions.

It is to be observed that a solid metal furnace tube, such as a platinum furnace tube, can be used in place of a metal-film-coated furnace tube. A solid platinum furnace tube is impervious to mobile ions, such as mo bile positive sodium ions, at a temperature of l,lO0 Centigrade. The inner wall of a solid platinum furnace tube is, of course, composed of a metal.

As shown in FIG. 1, a battery 18 is attached to the end of the platinum film 14 by means of a lead 16. The edge of the platinum film 14 will reach a temperature of only about 100 Centigrade, since it is not directly under the radiant heating coil 30. The battery 18 applies a positive 500 volts potential, with respect to ground, to the 700-Angstrom-thick platinum film 14. By applying a positive potential to the platinum film 14 on the furnace tube l2, one can further hinder the passage of mobile positive sodium ions from the outside of the tube 12 into the interior of the furnace tube 12.

A silicon wafer holder 22 is laid within the platinum coated quartz furnace tube 12. A silicon wafer 24, having a 1,200-Angstrom-thick silicon oxide insulator layer 26, is held by the silicon wafer holder 22 within the platinum coated quartz furnace tube 12. The resistance heating coil 30 is placed around the outside of the platinum-coated quartz furnace tube 12.

The platinum film 14 is held at a positive 500 volts with respect to ground. I-Ieat from the radiant heating coil 30, which is driven by an A. C. power source 34, raises the temperature of the silicon wafer 24 within the center of the platinum-coated quartz furnace tube to 600 Centigrade. A 100 cc/minute flowing oxygen gas from an oxygen container is passed through 95 Centigrade water 32 within a container 33, and then through the platinum-coated quartz furnace tube 12 While it is being heated, for 60 minutes. In accordance with the present invention, a 14-fold reduction in the number of mobile positive ions, including mobile positive 'sodium ions, within the silicon oxide insulator layer 26 is achieved, using a positively-charged platinum-coated quartz furnace tube 12, over what can be obtained using an uncoated quartz furnace tube. A three-fold reduction in the number of mobile positive ions, including mobile positive sodium ions, is achieved with the use of a positively-charged platinum-coated quartz furnace tube 12, over what can be obtained with the use of an uncharged platinum-coated quartz furnace tube 12.

FIG. 4 shows a processing apparatus for oxidizing semiconductor material in a mobile-positive-ion-free furnace tube 40. As shown in FIG. 4, a platinum-coated quartz furnace tube 40 is positioned by a suitable means within a resistance heating coil 42. A 700- Angstrom-thick platinum film 44 is evaporated upon the inside surface 45 of the quartz furnace tube 40. Oxygen gas is passed through the quartz furnace tube 40 from an oxygen container 48 at the rate of 300 cc/minute. Radiant heat, from the resistance heating coil 42, which is driven by an A. C. power source 50, raises the temperature inside the quartz furnace tube 40 to l, l 00 Centigrade. A positive potential of.+ 500 volts from a battery 60 is attached to the platinum film 44 by means of a lead 62, with respect to ground. The oxygen gas in the platinum-coated quartz furnace tube 40 grows a relatively mobile-positive-ion-free silicon oxide insulator layer on a silicon wafer 72, which is placed on a silicon wafer holder 74 in the platinum-coated quartz furnace tube 40. An exit port allows the oxygen gas 46 to exit from the quartz furnace tube 40. An l,lO0- Angstrom silicon oxide insulator layer 70 is grown upon an n-type silicon wafer 72, having p-type source and drain regions diffused therein, by oxidizing it for I80 minutes. It is found that the silicon oxide insulator layer 70 which is produced has a ten-fold reduction in the concentration of mobile positively-charged ions therein over a silicon dioxide insulator layer that is produced in a quartz furnace tube which does not have a platinum film thereon. A two-fold reduction in the concentration of mobile positive ions is produced therein from a silicon dioxide insulator layer produced in a platinum coated quartz furnace tube 40 which does not have a positive potential applied to the platinum film 44.

FIG. 5 shows an MOS field effect transistor 100 having a mobile-positive-ion-free silicon dioxide insulator layer 70 therein. As shown in FIG. 5, the silicon dioxide layer 70 is selectively etched so as only to extend from the edge of the p-type source region'90 to the edge of the p-type drain region 92.

A small area of a partially processed silicon'wafer 72 can then be fabricated-into a completed metal-silicon oxide-silicon (MOS) field effect transistor 100, as shown in FIG. 5. A gate electrode 102, such as an aluminum gate electrode, is deposited by vacuum evaporation'upon the silicon oxide layer 70 through an evaporation mask. A sourceelectrode 103 and a drain electrode 105 are attached to the p-type source region 90 and to the p-type drain region 92, also by vacuum deposition. A metal-silicon oxide-silicon (MOS) field effect transistor is thereby produced.

Due to the formation of .the silicon oxide insulator layer 70' within a mobile-positive-sodium-ion-free environment, mobile positive sodium ions are not trapped within the silicon dioxide insulator layer 70 of the MOS field effect transistor 100. The mobile-positive-sodiumion-free silicon dioxide layer 70, therefore, aids in producing an MOS field effect transistor 100, which begins to conduct a source-drain current, at asmall -3 volts threshold voltage, from a battery 110. The amount of source-drain current from the battery 112 also does not appreciably drift for a given gate voltage, under the pe riodic operation of the MOS field effect transistor 100. That is, the processing apparatus of the present invention aids in producing a silicon dioxide insulator layer 70, upon a silicon wafer 72, in such a way as to retard the trapping of mobile positive sodium ions within the silicon dioxide insulator layer 70 of the MOS field effect transistor 100. The decreased drift in the sourcedrain current for a given gate voltage, in the MOS field effect transistor 100, is due to the near lack of mobile positive sodium ions in the silicon dioxide insulator layer 70. If sodium atoms could migrate within the silicon dioxide insulator layer 70, a greater negative threshold voltage than -3 volts would be required, the added gate voltage being necessary to make up for the charge concentration of positive sodium atoms in the silicon dioxide insulator layer 70.

What is claimed is:

l. A process for growing a substantially mobile positive ion free silicon dioxide insulator layer of an insulated gate field effect transistor upon a silicon wafer, comprising:

placing the silicon wafer in a furnace tube having an interior wall coated with a thin non-oxidizing, highmelting-point film of material which will not melt in a temperature range of from 600 C to I,200 C and having a thickness of at least approximately 600 A which is impervious to mobile positive ions wherein-the material of the film is selected from the group consisting of platinum, rhodium, tantalum overcoated with silicon nitride and titanium overcoated with silicon nitride; applying a positive potential to said film to further hinder mobile positive ions from entering the interior of said furnace tube and from entering the silicon dioxide layer during its growth; passing an oxidation gas through said closed furnace tube to oxidize said silicon wafer to form a substantially mobile-positive-ion-free silicon dioxide insulator layer upon said silicon wafer and to flush the interior of said furnace tube of mobile positive ions; and heating said silicon wafer to a temperature of approximately I,100 Centigrade to increase its oxidation rate and to keep any mobile positive ions in the furnace tube in amobile state. I 2. A process of annealing and reducing the mobile positive ion concentration of an insulator layer of silicon oxide disposed on a wafer comprising:

placing the wafer having the silicon oxide insulator layer disposed thereon in a furnace tube which is coated with a thin non-oxidizing high-meltingpoint film of material which is impervious to mobile positive ions, the material of the film being selected from the group consisting of platinum, rhodium, tantalum overcoated with silicon nitride and titanium overcoated with silicon nitride, and having a thickness of at least approximately 600 A; passing an annealing gas through said closed furnace tube to anneal said insulator layer and to flush the interior of said furnace tube of mobile positive ions; and heating the silicon oxide insulator layer to a temperature of approximately 600 Centigrade to drive mobile positive ions out of said insulator layer into said annealing gas to anneal said insulator layer. 3. The process of claim 2 and also including after placing the wafer in the furnace 'tube and before passing the annealing gas therethrough the additional process step of:

applying a positive potential to said film to further hinder mobile positive ions from entering the interior of said furnace tube. 4. The process of claim 2 wherein: the annealing gas is oxygen which is caused to flow through water heated to a temperature of about C prior to the oxygen gas being introduced into the furnace tube. 5. The process of claim 3 wherein: the material of the thin non-oxidizing, high-meltingpoint film is platinum and the film has a thickness of from approximately 600 A to 6,000 A. 

2. A process of annealing and reducing the mobile positive ion concentration of an insulator layer of silicon oxide disposed on a wafer comprising: placing the wafer having the silicon oxide insulator layer disposed thereon in a furnace tube which is coated with a thin non-oxidizing high-melting-point film of material which is impervious to mobile positive ions, the material of the film being selected from the group consisting of platinum, rhodium, tantalum overcoated with silicon nitride and titanium overcoated with silicon nitride, and having a thickness of at least approximately 600 A; passing an annealing gas through said closed furnace tube to anneal said insulator layer and to flush the interior of said furnace tube of mobile positive ions; and heating the silicon oxide insulator layer to a temperature of approximately 600* Centigrade to drive mobile positive ions out of said insulator layer into said annealing gas to anneal said insulator layer.
 3. The process of claim 2 and also including after placing the wafer in the furnace tube and before passing the annealing gas therethrough the additional process step of: applying a positive potential to said film to further hinder mobile positive ions from entering the interior of said furnace tube.
 4. The process of claim 2 wherein: the annealing gas is oxygen which is caused to flow through water heated to a temperature of about 95* C prior to the oxygen gas being introduced into the furnace tube.
 5. The process of claim 3 wherein: the material of the thin non-oxidizing, high-melting-point film is platinum and the film has a thickness of from approximately 600 A to 6,000 A. 